Isolation systems such as vehicle suspensions or the like serve to reduce the shock and vibratory forces between relatively movable members such as a vehicle body (a sprung mass) and a support or vehicle wheel (an unsprung mass). Forces transmitted between the members are attenuated using dampers that comprise piston and cylinder assemblies having variable volume chambers interconnected by an orifice or passageway through which fluid is displaced, and throttled, according to the size of the orifice.
Prior art isolation systems may be characterized as passive, fully active or semiactive. A passive isolation system, such as the spring-dashpot combination used in most automobiles, typically provides good isolation in a certain frequency range of operation, but is subject to amplified excitation in passing through its resonant or natural frequency such that its damping forces will at times amplify, rather than attenuate, force transmission between the interconnected (sprung and unsprung mass) members. More particularly, a passive isolation system is often incapable of providing damping forces sufficient to appropriately attenuate the excitation of the sprung mass member. Other passive isolation systems provide good control of the sprung mass isolation at the natural frequency of the suspension, while imparting too much damping force between the interconnected members at other frequencies, resulting in a "harsh" ride.
Fully active isolation systems differ from passive systems in that they employ an external power source or force generator for supplying energy in a controlled manner to counteract vibrational forces. As a result, the damping coefficient and hence the effective natural frequency of a suspension employing a fully active damper can be continually adjusted to the desired value to provide isolation superior to that of passive devices. However, fully active systems require a large auxiliary power source and are not sufficiently responsive at high operating frequencies due to inadequacies of such equipment to respond rapidly to control signals.
Semiactive isolation systems are defined as those which require no external energy other than that needed to actuate valves, sensors and controls, yet are capable of providing for rapid changes in the damping coefficient of the damper interconnecting the members so as to optimize the attenuation of forces between the members. A semiactive system can create a force opposing motion, but not in the direction of motion. Thus, the term "semiactive" refers to control systems which are only capable of removing energy from a system. Semiactive systems are nonetheless capable of performance approaching that of a fully active system when operated pursuant to a suitable control policy, and in particular those control policies which emulate a hypothetical "skyhook" damper as described in Karnopp, D. C. et al., "Vibration Control Using Semiactive Force Generators," ASME Paper No. 73-DET-123 (June 1974), incorporated herein by reference. Semiactive dampers and control policies for them, are disclosed in Karnopp, U.S. Pat. No 3,807,678; Miller et al., U.S. Pat. Nos. 4,821,849, 4,838,392 and 4,898,264; Boone, U.S. Pat. No. 4,936,425; and Ivers, U.S. Pat. No. 4,887,699, all owned by the assignee of the present invention. The disclosures of the foregoing patents are incorporated herein by reference.
Semiactive dampers may be either of the "off/on" type, of the "orifice-setting" type, or of the "force-controlled" type. An "off/on" semiactive damper is switched, in accordance with the dictates of a suitable control policy, between alternative "on" and "off" damping states or conditions. In the "on" state, the damping coefficient of the damper is of a preselected, relatively high magnitude. The term damping coefficient as used herein is understood to mean the relationship of the damping force generated by the damper to the relative velocity across the damper, which relationship is not necessarily linear. In its "off" state, the damping coefficient of the damper is approximately zero or of a relatively low magnitude sufficiently greater than zero so as to discourage "wheel hop". An orifice-setting semiactive damper is also switched during operation between an "off" state, where the damping coefficient is approximately zero or of some relatively low magnitude, and an "on" state. However, when a orifice-setting semiactive damper is in its "on" state the damping coefficient thereof may be and normally is changed between a large (theoretically infinite) number of different magnitudes. The magnitude of the damping coefficient is typically determined by the diameter setting of the valve orifice in the damper.
A "force-controlled" damper, in theory, is capable of creating any desired dissipative force in the "on" state independent of the relative velocity across the damper. This is in contrast to the aforementioned "off/on" and "orifice-setting" dampers in which the damping force in the "on" state depends on the relative velocity across the damper. A force-controlled damper can either be realized by use of feedback control or by use of pressure control valves. In the "off" state the force-controlled damper will command the valve to the full-open position in which the damping coefficient is approximately zero or of some relatively low value.
Although the foregoing semiactive isolation systems offer significant performance advantages over other types of isolation systems they have been known to experience difficulties when subjected to large, abrupt input disturbances such as those encountered on rough terrain or upon the landing of an aircraft, for example. Excessive vehicle body motions and suspension travel can often result in damaging or uncomfortable force inputs to the vehicle when the suspension reaches its end of travel in either a retracted or extended condition so as to impact the mechanical end stops of the suspension. The aforesaid suspension end-stop collisions are often referred to as "bottoming out" of the suspension, when such occurs in the retracted condition. End-stop collisions result in degraded isolation of the vehicle body by significantly increasing the root-mean-square (RMS) accelerations thereof and further place undue stress on system components to such an extent as to shorten their useful life.
Semiactive isolation systems employing an above-described "skyhook" control policy or a derivative thereof, tend to increase the average range of suspension deflection to provide "smoother" ride characteristics and therefore can, under certain conditions, actually increase the incidence of suspension end-stop collisions. This tendency is discussed in Miller, "Tuning Passive, Semiactive and Fully Active Suspension Systems," Proceedings of the 27th CDC of IEEE, Vol. 3, 1988 and in Ivers et al., "Experimental Comparison of Passive, On/Off Semiactive and Continuous Semiactive Suspensions" SAE Paper No. 892484, Dec. 7, 1989.
The incidence of suspension end-stop collisions could be reduced or even eliminated by use of a very stiff damper with a high damping coefficient. However, this would defeat the performance advantages of semiactive control by unnecessarily limiting the range of suspension deflection for the given range of motion of the suspension and unacceptably degrade the isolation of the vehicle.
Various types of spring and elastomeric bumpers have been designed to reduce the severity of end-stop collisions. Typically, these bumpers or other means are designed to be internal to the damper cylinder and provide for hydromechanical or pneumatic cushioning after the piston has approached closer to a particular end-stop than a prescribed distance. Examples of such devices are disclosed in Mourray, U.S. Pat. No. 4,527,674 and Kaneko, U.S. Pat. No. 4,700,611. Other arrangements are provided which progressively increase damping as the damper nears its end of travel, such as disclosed in Sorgatz et al., U.S. Pat. No. 4,004,662; Wossner, U.S. Pat. No. 4,768,629; and Hauswirth, U.S. Pat. No. 3,885,654.
A limitation of the aforementioned stroke dependent dampers is that they tend to reduce the available range of suspension deflection which adversely affects their ability to provide optimized isolation of the vehicle. Also, the end-stop cushioning or other means is permanently installed in the damper and may not be overridden, rendering the damper unsuitable for the cooperative implementation of semiactive control functions. The aforementioned dampers further are limited in that they do not anticipate and avoid end-stop collisions according to variable quantities external to the damper such as mass, velocity and input displacement. All of the above result in reduced isolation performance.